1. Field of the Invention
The present invention relates to a medicinal plaster for improved skin absorption of dermally applied medication.
2. Description of the Background
It is known that systemic medicinal effects can be obtained with medicinal plasters or so-called therapeutic plaster systems, designated lately as transdermal therapeutic systems. At present, this type of system is used in connection with the drug scopolamine for kinetosis, nitroglycerine for coronary heart disease and clonidine for hypertension, as well as for transdermally administered estrogens. However, the plaster systems used to date are technically complex and the attainable absorption rates, measured through the systemic drug concentration in the blood, are clearly less than those after oral administration. In addition, the systems show considerable variation from patient to patient with regard to the determined serum concentrations of the administered drugs.
The plaster systems entail diffusion units in which the medications are released by diffusion at controlled rates from a mechanically fixed drug reservoir, usually tissue tolerant polymers. The systems used are currently divided into membrane systems, i.e., membrane plaster and matrix systems. In the membrane systems the drug, after release from the carrier substance, must permeate a membrane, which serves as a control element for the constant absorption rate. Thereby, it is possible to attain a release characteristic, which approximately corresponds to pharmacokinetics of zero order. In matrix systems, the drug stored in depot form diffuses directly from the polymer matrix into the skin.
The transfer of medication from the plaster system into the skin occurs according to the laws of diffusion, quantified in the diffusion principles according to Fick: ##EQU1## whereby per time unit (t) the drug amount transported (Q), the diffusion rate, is dependent on the diffusion coefficient (D), the exchange surface (F) and the concentration difference (C.sub.1 -C.sub.2) as well as the diffusion distance or the layer thickness (d). It is observed that plasters with mechanically rigid matrices do not optimally follow the diffusion conditions.
Thus, the polymer matrices and biological membranes such as irregularly formed complementary skin surface, which represent the exchange surface, adhere such that there is an incomplete utilization of the biologically available absorption surface. Simultaneously, the diffusion distance is thereby increased in several areas of the adhering absorption surface of the plaster. Both effects mean a deterioration of the general diffusion conditions. As, in addition, the speed of the diffusion process also depends on the temperature, the temperature exchange between the technical resorption area and the drug reservoir on the one hand, and the skin surface on the other hand, is not optimally attained with the incomplete superposition. Another disadvantageous effect is the relatively slow water absorption which is needed for the dissolving process of the medication.
In addition, the production of membrane and matrix systems is technically costly and requires special apparatus, which causes higher costs than for the production of oral forms of application.
Accordingly, a need clearly exists for a relatively simple and inexpensive means by which dermally applied medications can be applied with excellent skin absorption.